1. Field of the Invention
This invention relates to materials and processes for deposition of conformal films containing metals on solid substrates, and in particular, to films including copper, cobalt and iron metals or their oxides or nitrides. This invention may be applied to the fabrication of microelectronics devices.
2. Description of the Related Art
As the speed and functionality of semiconductor microelectronic devices are improved, new materials are needed. For example, materials with higher electrical conductivity are needed to form the wiring between transistors in integrated circuits. Copper has higher electrical conductivity and better stability against electro-migration than does aluminum. Therefore, copper is becoming more commonly used in silicon semiconductors. This trend is described in the International Technology Roadmap for Semiconductors, published on the Internet at http://public.itrs.net/Files/2001ITRS/Home.htm.
Copper interconnections must also be disposed conformally in structures, such as narrow holes, and the resulting films must have highly uniform thickness. If there are variations in thickness, the electrical conductivity of the copper in a trench or via is degraded because of increased electron scattering from the rough surface of the copper. Thus high-quality barrier/adhesion layers desirably have very smooth surfaces.
One method that is suitable for making smooth, conformal layers is “atomic layer deposition”, or ALD (also known as atomic layer epitaxy). The ALD process deposits thin layers of solid materials using two or more different vapor phase precursors. The surface of a substrate onto which film is to be deposited is exposed to a dose of vapor from one precursor. Then any excess unreacted vapor from that precursor is pumped away. Next, a vapor dose of the second precursor is brought to the surface and allowed to react. This cycle of steps can be repeated to build up thicker films. One particularly important aspect of this process is that the ALD reactions are self-limiting, in that only a certain maximum thickness can form in each cycle, after which no further deposition occurs during that cycle, even if excess reactant is available. Because of this self-limiting character, ALD reactions produce coatings with highly uniform thicknesses. Uniformity of ALD film thicknesses extends not only over flat substrate surfaces, but also into narrow holes and trenches. This ability of ALD to make conformal films is called “good step coverage.”
ALD of copper has been demonstrated from the copper precursor Cu(II)-2,2,6,6-tetramethyl-3,5-heptanedionate by P. Martensson and J.-O. Carlsson in the Journal of the Electrochemical Society, volume 145, pages 2926-2931 (1998). Unfortunately, copper from this ALD process only grows on pre-existing platinum surfaces, and does not nucleate or adhere to most other surfaces in the temperature range (<200° C.) in which there is a true self-limiting ALD process. Other reactions have been suggested for ALD of copper, but no data have been published to demonstrate that the proposed surface reactions are actually self-limiting. Therefore it would be highly advantageous to have an ALD process for copper that nucleates and adheres to surfaces other than platinum.
U.S. Pat. No. 6,294,836 reports improvement in the adhesion of copper by use of a “glue” layer of cobalt between the copper and a substrate. However, known chemical vapor deposition (CVD) techniques for depositing cobalt have poor step coverage, giving only 20% thickness at the bottom of a hole with aspect ratio 5:1, according to U.S. Pat. No. 6,444,263. ALD of cobalt has been claimed in US Patent Application No. 2002/0081381 for the reaction of cobalt bis(acetylacetonate) [Co(acac)2] with hydrogen, but no step coverage data were given and growth was seen only on pre-existing iridium surfaces. US Patent Application No. 2002/0081381 also claims non-selective growth of cobalt by the reaction of Co(acac)2 with silane, but this cobalt may be contaminated with silicon. Thus it would be advantageous to have a deposition process for pure cobalt having high step coverage.
Thin layers of copper and cobalt are also used to form magnetoresistant write and read heads for magnetic information storage. These layers need to have very uniform thicknesses and very few defects or pinholes. While successful commercial processes exist for making these devices, it would be advantageous to have deposition processes for copper and cobalt that produced layers with more uniform thickness and fewer defects.
Advanced designs for magnetic memory integrated with microelectronic circuits (see, for example, US Patent Application No. 2002/0132375 and U.S. Pat. No. 6,211,090) call for highly uniform and conformal layers of metals (particularly Fe, Co, Ni, Cu, Ru, Mn) with tightly controlled thickness and sharp interfaces. There are no known methods for depositing these metal layers with the required conformality and control of thickness.